Hermitian Operators: Homework Equations & Attempt at a Solution

In summary, the student is struggling to solve a problem involving the expectation value of a square operator. They have first done the steps of expressing the expectation value explicitly and splitting the operator into its matrix elements. They then use a clever method to treat the operator as a Hermitian wavefunction. Next, they need to think about the action of each matrix element on the wavefunction. After doing that, the problem seems easy.
  • #1
kehler
104
0

Homework Statement


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Homework Equations


sqp8q1.jpg



The Attempt at a Solution


I've gone round in circles doing this! I started of by writing it as an integral of (psi* x A_hat2 x psi) w.r.t dx, then using the equation above but I keep coming back at my original equation after flipping it around. Are there any tricks in particular that I should know when dealing with squares of operators? Or any assumptions that I need to make? Please help :(
 
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  • #2
It may help to insert a "resolution of the identity" (as Shankar calls it). The operators can be treated in terms of their "matrix elements" on the Hilbert space, which is the space of the functions, Ψ, on which the operators act.

Are you familiar with the Dirac (bra-ket) notation? I would start with Dirac notation, and then simply insert the appropriate identity resolutions.
 
  • #3
What's a resolution of the identity? I don't have the Shankar text. We use Griffiths in class... No unfortunately I'm not too familiar with Dirac Notation :(. I know roughly what it is but I'm not confident in using it just yet. So far we've just been taught the integral form. I think we're just meant to manipulate the equation till we get to be the required function but I've tried that several times without getting anywhere
 
  • #4
OK. The first thing to do is to express the expectation value explicitly. Then, split A2 into factors of A. Then, use your definition for Hermiticity in a clever way.
 
  • #5
I've done the first two steps. It's the third that I'm having trouble with.
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What do I do from here? :s
 
  • #6
You didn't really do the second step, then. Think of the action of each A independently. That is, think of the action of the first A on the wavefunction as producing a new wavefunction, and then the other A acts on this new wavefunction.
 
  • #7
Ahh ok I see :). Thanks. Gosh it seems really easy now.
 

What is a Hermitian operator?

A Hermitian operator is a mathematical operator in linear algebra that has the property of being self-adjoint. This means that the operator is equal to its own conjugate transpose, and therefore has real eigenvalues.

What are some common examples of Hermitian operators?

Some common examples of Hermitian operators include the position and momentum operators in quantum mechanics, as well as the Hamiltonian operator which represents the total energy of a quantum system.

What are the key properties of Hermitian operators?

Some key properties of Hermitian operators include having real eigenvalues, being self-adjoint, and having orthogonal eigenvectors. They also have the property that the expectation value of a Hermitian operator is always a real number.

How do you determine if an operator is Hermitian?

An operator is Hermitian if it is equal to its own conjugate transpose, meaning that the operator must satisfy the condition A = A†. This can be checked by computing the conjugate transpose of the operator and comparing it to the original operator.

How are Hermitian operators used in quantum mechanics?

Hermitian operators are fundamental in quantum mechanics as they represent physical observables of quantum systems. They are used to calculate the probabilities of various outcomes in experiments, as well as to determine the time evolution of quantum states.

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